Glass sheets are quenched to provide tempering or heat strengthening resulting from internal stress resulting from a temperature gradient within the glass during the cooling. Quenching increases a mechanical strength in the glass and hence provides an increased resistance to breakage as apparent to annealed glass which has a minimal internal stress. In tempering, quenching gas or other coolant is impinged upon opposite surfaces of the glass sheet to provide rapid cooling so that the final cooled glass sheet has compressive forces at its surface and tensile forces at its center making it stronger and more resistant to breakage. Tempered glass also has a characteristic of breaking into relatively small pieces as compared to the sharp shards which result when annealed glass is broken.
With heat strengthening, quenching gas is also impinged on opposite surfaces of the glass sheet but at a much lower rate and resulting in less compressive forces than tempered glass. Both tempering and heat strengthening, can be performed on flat or curved glass sheets conventionally used for architectural purposes or for vehicle windows.
From a safety standpoint, it is important that when tempered or heat strengthened window glass breaks no large, sharp splines form which could seriously harm a person. Various motor vehicle regulating authorities specify the maximum spline length resulting when a passenger vehicle window fractures. Small spline sizes are relatively easy to achieve with a thick window glass but when thickness is approached five millimeters or less, spline length begins to increase to the unacceptable range 20-30 millimeters depending upon the particular standard. In addition to regulating the maximum shard size, the minimum particle size is also regulated. While large splines pose a danger of laceration, small particles pose a potential danger of ingestion. The requirements of various regulatory agencies is described in U.S. Pat. No. 4,128,690, 4,178,414 and 4,182,619 which are incorporated herein by reference.
In order to achieve a uniform particle size upon glass breakage without having small ingestable particles or large sharp splines glass, quenching devices have been developed which utilize arrays of nozzles which cool a series of points across the glass surface. The resulting non-uniform stress distribution across a glass surface helps to prevent the formation of long splines.
Glass sheet quenching devices conventionally include apposed blastheads oriented on opposite sides of the glass sheet to be quenched. Each blasthead is provided with a plenum housing which is supplied with a pressurized quenching gas and has a plurality of nozzles for directing discrete jets of coolant against the glass surface. A similar blasthead structure specifically designed to quench glass while horizontally supported on a roller conveyor is shown in U.S. Pat. No. 4,515,622 which is incorporated by reference herein.
When quenching glass sheets, it is important to maintain uniform cooling gas flow and pressure so as to prevent any significant variation in pressure over the surface of the glass sheet being quenched or a variation from sheet to sheet. One of the factors having to cause cooling variation from nozzle to nozzle is the influence of the rollers upon which the glass rests. The presence of the rollers tend to hinder the flow of gas to points cooled adjacent thereto. Points cooled while located adjacent a roller tend to be non-symmetrical. Blasthead nozzles are designed in such a manner so that roll shrouding affects are minimized, however, there remains significant room for further improvement.